genirq/msi: Make the .prepare callback reusable

The .prepare callbacks are so far only called from msi_domain_alloc_irqs.
In order to reuse that code, split that code and create a
msi_domain_prepare_irqs function that the existing code can call into.

Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
diff --git a/include/linux/msi.h b/include/linux/msi.h
index f71a25e..1c0bb2c 100644
--- a/include/linux/msi.h
+++ b/include/linux/msi.h
@@ -279,6 +279,10 @@
 int platform_msi_domain_alloc_irqs(struct device *dev, unsigned int nvec,
 				   irq_write_msi_msg_t write_msi_msg);
 void platform_msi_domain_free_irqs(struct device *dev);
+
+/* When an MSI domain is used as an intermediate domain */
+int msi_domain_prepare_irqs(struct irq_domain *domain, struct device *dev,
+			    int nvec, msi_alloc_info_t *args);
 #endif /* CONFIG_GENERIC_MSI_IRQ_DOMAIN */
 
 #ifdef CONFIG_PCI_MSI_IRQ_DOMAIN